This invention relates to overcharge preventive devices for preventing, or for issuing an alarm for, an occurrance of overcharge of automotive batteries.
FIG. 1 shows a schematic circuit diagram of a typical automotive battery charging device. A battery 1 is coupled, via a main fuse 2 and a key switch 3, across an electric load 4 and the ignition system of an automotive engine. Further, an alternator (AC generator provided with a full-wave rectifier) 5 is coupled across the battery 1 via the fuse 2. When the key switch 3 is made, the voltage of the battery 1 is applied across a starter motor (not shown) such that the engine is started. The alternator 5 is then driven by the started engine, to supply the output voltage to the battery 1 and the load 4.
This charging circuit, however, has the following problem. When the voltage regulator of the alternator 5 malfunctions and the output voltage of the alternator 5 rises above a predetermined level, an abnormally high voltage is applied across the battery 1 and the load 4. The main fuse 2 usually has the rating of 60 to 80 amperes in accordance with the overall load. Thus, in the case where the output voltage of the alternator 5 rises abruptly to a very high level, a large current flows into the battery 1 and melts the fuse 2, thereby preventing the overcharge of the battery 1. When, however, the alternator 5 and the battery 1, etc., are at a high temperature, or when the rpm of the enigne is low, or when a multitude of loads 4 are in use, the level of current that flows into the battery 1 remains relatively low and the fuse 2 remains unmelted, such that the battery 1 is gradually overcharged. This results in an occurrance of leakage of the liquid contained in the battery, the corrosion of the hood of the automobile due to the gas generated from the battery 1, and failures of the electric equipment due to the application of high voltage to the electric load 4.
FIG. 2 shows in detail a charging circuit provided with a conventional overcharge alarm device. A battery 1 is coupled across an alternator (charging AC generator) 2, and, via a switch 3, across a load 4. The outputs of the armature windings 5 of the alternator 2 are coupled to the full-wave rectifier circuit 6, the first output terminal 61 of which is coupled directly to the positive terminal of the battery 1, while the second output terminal 62 of which is coupled to the positive terminal of the battery 1 via a charge indicator lamp 7 and the key switch 8, as well as to a terminal of the field winding 9. A voltage regulator 10 is coupled to the two terminals of the field winding 9 to regulate the field current flowing through the field winding 9. The input of the regulator 10 is coupled to the positive terminal of the battery 1 via the key switch 8. A voltage detector 11 coupled across the load 4 detects the voltage applied thereacross. An alarm sound generator 12 generates an alarm sound when the voltage detected by the voltage detector 11 rises above a predetermined level.
The method of operation of the circuit of FIG. 2 is as follows. When the key switch 8 is turned on, the voltage across the battery 1 is applied across the starter motor (not shown), thereby starting the engine. The charging generator 2 is thus driven to generate an output voltage, and thereby charges the battery 1 and supplies power to the load 4 via the switch 3 which is made together with the key switch 8. The output voltage of the charging generator 2 is controlled within a predetermined range by the operation of the regulator 10. Thus, in the normal operation, the voltage detector 11 does not detect an abnormally high voltage and the alarm generator 12 is not activated. On the other hand, when an abnormally high voltage is generated by the charging generator device 2 due to the failure of the regulator 10, etc., this abnormally high voltage is detected by the voltage detector 11, and an alarm sound is generated in response thereto by the alarm generator 12, thereby notifying the driver of an occurrance of an abnormal state.
By the way, before the engine is started, the current from the battery 1 flows through the charge indicator lamp 7 to turn it on. On the other hand, after the engine is started, the voltage levels at the first and the second regulator outputs 61 and 62 become balanced, and hence the lamp 7 is turned off.
The circuit of FIG. 2 still has this problem: The provision of the alarm sound generator 12 results in an increased production cost and a requirement for a larger installation space.